Are primary sources always credible? why or why not?

Just because a source is primary, doesn't always mean that it is credible.  Just as in present day, people and institutions have inherent biases that can potentially render a source unusable for your research.  Intended audience, purpose of material (was it generated for propaganda purposes?), and one-sided viewpoints can indicate whether or not a source is suitable to use.  

i think it was because of how protected egyptian civilization was was which allowed them to evolve without much outside influence.